The present invention relates generally to implantable medical devices and more particularly to controlling surface properties of implantable biocompatible materials suitable for fabrication of implantable medical devices.
Various materials have been used for the purpose of substituting or supporting organic functions of the human body, ranging from metals to ceramics to polymers. Almost all materials currently used are still being optimized in terms of composition and processing. In recent times, the focus has been aiming more and more toward the surface properties of the materials. Since body tissue normally interacts only with the top few nanometers of an implanted material, the chemical and topographic properties often determine the success or failure of an implantation process. Depending on the intended use of an implant, various topographies may be required. The desired surface may be very smooth as in the case of implants directly in contact with the blood flow (e.g. artificial heart valves), a structured surface with very high roughness as in the case of permanent implants where good adhesion and quick cell ingrowth are important (e.g. shafts of hip implants) or surface structures with intermediate roughness. In the case of vascular implants, it has been found that an optimum surface roughness may be beneficial to promote endothelial cell monolayer coverage.
Commonly used techniques for modifying surfaces include chemical treatments, laser structuring as well as mechanical surface treatments. A simple, widespread surface modification technique, with which an increase in surface roughness can be achieved, is grit blasting. An increased roughness may provide both improved adhesion properties and a favored basis for cell growth. Accordingly, grit blasting is applied for permanent implants which require a consolidated ingrowth, e.g. shafts of hip implants and dental implants. The technique is comparatively easy to perform and applicable for large quantities. However, for grit blasted implants it is known that a risk of remaining particles exists. Another limitation results from the fact that mechanical stresses are imposed on the material. When applied to sensitive structures, grit blasting carries the risk of deforming the workpiece. For titanium implants chemical and plasma chemical surface modifications are subject of current research with promising results. In this case highly reactive, mostly fluorine containing chemicals are used in order to etch the surfaces. Similar to grit blasting, with this method the surface roughness can be increased resulting in promoted cell ingrowth.
Another microstructuring method is offered by laser ablation. Using this method three dimensional structures can be created. These structures may be used to promote cell ingrowth; however an even more important feature offered by these structures is the possibility to load the surface with therapeutic substances. This way drugs can be applied directly into the affected location, resulting in a high therapeutic efficacy at small amounts of drugs needed. Examples for drug coatings are antibiotics, antithrombotic agents as well as cell growth stimulants. Other microstructuring techniques that give the possibility to create three dimensional structures originate mainly from the field of microchip fabrication. Photo and laser-lithographic techniques are employed by depositing a protection layer of lacquer on the substrate. Subsequently the features are etched electrochemically into the material. The technique may be time consuming and very difficult to apply for complex geometries like cardiovascular stents.
Accordingly, there is a need for a microstructuring technique that combines the advantages of a selective three dimensional structuring with the convenience of a chemical method for implantable medical devices.